Didn't Marvin Hagler fight the very last scheduled 15-rounder? I recall a lot of angst when Leonard outpointed him (allegedly!!) as Hagler would likely have stopped him over 15 (of course Leonard paced himself excellently for the 12 rounds, not 15, but there were some arguments I think)

I know that Joe Louis fought the very last scheduled 20 rounder, though he whacked out Buddy Baer much sooner than that.